Twitter sent Meta a cease-and-desist letter over the newly launched Threads app, sources familiar with the letter's existence told ABC News.

The letter was sent by Twitter's legal team Thursday, the sources said.

The letter accused Meta of misappropriating Twitter's trade secrets and said Meta hired former Twitter employees who retained proprietary information, the sources said.

Threads, which is built off the existing Instagram app, has been extremely popular in its first 24 hours since release.

Meta CEO Mark Zuckerberg said Thursday morning the app had already signed up 30 million users.

Social media stars began flocking to the platform late Wednesday when it launched. Kim Kardashian had amassed 1.7 million followers by Thursday afternoon, while singer Shakira had 1.4 million and Zuckerberg himself also had 1.4 million as of Thursday afternoon.

Threads has been seen as a top rival to Twitter, which was purchased by Elon Musk in October 2022 for roughly $44 billion, and has been embroiled in controversy ever since.

In his latest controversial decision, Musk announced July 1 that Twitter would be limiting the number of tweets people can view each day.

Twitter, in an effort to significantly slash costs, cut roughly 75% of its 7,500-person workforce not long after Musk acquired the company last year.

ABC News' Meredith Deliso contributed to this report.